The Consumer / Patient
08-2008
 The Prescription and Vision
It starts with the prescription. The prescription (Rx) describes the patient’s Refractive Error. Any Prism needed for the eyes to work together and an Add if needed for near vision/reading. (Fig. 1)
The prescription or lens formula is the starting point for recommending the best lens material and for guiding frame size and shape selection. The axis describes the meridian (or direction) of lens powers needed in the frame in front of the eye. Here’s how it works.
Zero degrees is always on the left of each of the patient's eyes, 90˚ is up and 180˚ is on the right. This is the same for both the right and left eyes and is called TABO notation. Axis is described in 1 degree steps from 1 to 180 degrees. There is no need to describe axes greater than 180 since everything that happens above the 180 line also happens below the 180 line. See Fig. 2. The eye may have a simple prescription, sphere power only or a compound prescription, a sphere and cylinder power. The position of the cylinder will be described by an axis.
 The prescription can be dissected to know where powers are located. This is helpful when determining how the Rx will look in the chosen frame size and shape.
See Fig. 3, the Rx R -2.00-2.00 x 90; L -3.50 sphere, when analyzed looks like this. Visit 2020mag.com for the activated version of the Opticians' Handbook. It will have a fuller explanation of dissecting the prescription, examples and ways to test yourself.
Who wears what kind of lens?
If the patient has no refractive error or does not need glasses, refractive condition is called emmetropia or normal vision (Fig. 4). If the Rx is minus, the patient is myopic (near sighted), the image is focused in front of the retina and requires minus lenses to move the image backwards onto the retina. Minus lenses are more curved on the back than the front, thinner in the center than the edge. For plus Rxs, the patient is hyperopic (far sighted). An image is focused behind the retina, lenses more curved on the front than the back, thinner at the edge than the center, and move the image forward onto the retina.
When the Rx is a sphere only and has no cylinder value the eye only needs one power for correction in all meridians. However, most Rxs (about 70 percent) have a cylinder value and are astigmatic. Therefore lenses will have two different powers; arranged perpendicular to each other to correct the two powers that are needed by the eye. The cylinder axis is the orientation of the two powers (1 to 180 degrees). If there is a cylinder power, there must be an axis. An astigmatic prescription may be any combination of plus, plano or minus powers.
 The Prescription in Eyewear
The optical center (OC) of the lens is located on the optical axis (Fig. 5). The OC is the point on the lens where there is no prism. At all other locations, light is bent and therefore at that point, has a prism value.
The eye has a visual axis; this describes the patient’s line of sight. When the eyes are looking far away (more than 20 feet), the visual axes are parallel and this is called the distance PD (interpupillary distance) (Fig. 6). In eyewear our goal is to align the optical axis of the lens with the visual axis of the eye. When correctly aligned, both the right and left lenses provide the best, and most importantly, binocular vision. Both eyes must work together and that means that the brain is able to make one merged image from the image of each of the eyes.
 The prism box (Fig. 7) on the Rx tells us whether the lenses will have the OC placed in front of the patient’s visual axis (PD) or be offset. If there is no prescribed prism, the OC is placed at the PD. If there is prescribed prism, then the point on the lens, with that amount of prism, is placed at the PD (Fig. 8). The result is a properly centered prescription on the eye.
Some time between the ages of 40 and 50, the inability to accommodate or reshape the crystalline lens to focus on near objects (i.e. reading) becomes difficult. This condition is called presbyopia and is the last condition described on the prescription (Add). The lens in the eye can no longer be re-shaped and becomes more plus, so an additional plus lens must be added for reading or close objects. The add value is a sphere power needed for clear near vision.
 The reading prescription is the sum of the distance prescription and the addition. It can be written by the doctor as a distance Rx with an add or as a SV Rx. In SV, it is the sum of the sphere power and add; the cylinder and axis stay the same. See (Fig. 9).
What Would You Do?PATIENT SITUATIONTwo young women enter your office, you guess that they are in their mid-twenties; Louisa has a new prescription for glasses and tells you that she wants the same look as her friend, Janet. Her prescription reads -5.50 sphere and a -6.25 – 0.50 x 180. She is about 5’10” and her friend is petite. Her friend also is wearing a pair of rimless lenses that slightly magnify her eyes. How can you use what you know so far to help you help Louisa? SOLUTIONFirst, Janet’s glasses magnify, that means that the lenses are plus. In this case, Louisa requires minus lenses. That means that her glasses will not look like Janet’s no matter what you do since the edges will be thicker than the centers. If Janet’s are plus, the centers are thicker than the edges. Also, Louisa is bigger than Janet and wears a larger frame than Janet. Larger lenses will be thicker so it will be hard to get a pair of glasses to look the same as far as lens thickness. However, rimless is an option for Louisa and there are many colors and shapes that you can suggest that will make a terrific pair of eyewear. Smaller sizes that look good is the target. What do you know? Louisa is near sighted in both eyes. Her left prescription tells you that she has a small amount of astigmatism at axis 180. For minus lenses, they are thicker at the edge than in the center. Thickness can be reduced by using smaller sized lenses and ones without sharp corners i.e., that are oval or round. Also, the better-centered Louisa’s eyes are in the lens shape, the thinner the lenses.
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